1. Field of the Invention (Technical Field)
The present invention relates generally to the field of instruments used for measuring distances. In particular, the present invention relates to instruments for measuring a length of an elongate line.
2. Background Art
In the well drilling industry cables or wires are lowered into the well for determining well depths. The lack of identifying features on a cable render measurement of the length of cable lowered into the well difficult to determine from the position of the wellhead, or well opening. An early method of measurement commonly referred to as “stringing in” utilized a predefined length of cord to measure the length of cable entering a well, one predefined length at a time, until the desired maximum depth was reached.
Since that time wheeled measuring tools and others, commonly referred to as “depthometers”, have replaced the stringing in method as a more efficient means for measuring line length. As used herein, the term “depthometer” refers to a measuring device used to measure the length of an elongate cable, wire, line or other elongate object (hereafter collectively referred to as a “line”). Conventional depthometers engage the line in some fashion such that the line passes through or alongside the instrument. As the line progresses through the instrument, the line rotates a wheel which in turn increments a registering mechanism that is calibrated to measure the length of the line in close approximation to the actual length passing through the instrument.
One such wheeled depthometer, manufactured by Cavins Company, was designed to replace the stringing in method and has seen widespread use in the oil and gas industry for decades. This depthometer engages the line at the wellhead, then is suspended in position by a tether or held in position with a stand mechanism. In this configuration the line is not translated beyond the relatively small area of the wellhead opening and the depthometer remains relatively fixed in position. The line passes over a wheel of the Cavins depthometer as it is lowered into the well causing the wheel to rotate, which in turn increments a counter that presents a length measurement output to the user.
Over the past few decades there have been significant changes to operating procedures, methods, and standards used by the well industry, in particular the oil and gas industry. To accommodate newer procedures, the line must often be passed through additional mechanisms at the wellhead before being lowered into the well. Also, safety regulations limit physical contact between users and lines lowered into a well, as well as with instruments or mechanisms attached to, or engaged with lines. There has not, however, been a concurrent alteration to the design of depthometers to comply with revised operating and safety procedures.
One difficulty that arises when using depthometers is the obstruction caused by the wellhead lubricator. A lubricator is a section of pipe large enough to contain the tools necessary to perform certain tasks inside of a well bore. Completed wells employ lubricators to contain gas pressure or fluids for safety purposes. Attached to the top of the lubricator is a mechanism commonly referred to as an “oil saver”, “pack off”, or “blow out preventer,” which contains a rubberized seal and can be compressed around the line into the well to contain gases and fluids. While depthometers are used on incomplete wells where gas and fluid containment may not be required, depthometers are most often used on completed wells in conjunction with a lubricator. The lubricator is generally at least twenty feet in height and is placed atop a typical wellhead, which is approximately five feet in height. A depthometer cannot be placed within the lubricator, therefore it must be placed at a height on the line above the wellhead that is beyond the grasp and visibility of the user.
To avoid the obstruction caused by the lubricator, depthometers are sometimes positioned in proximity to the draw-works drum used to take up and release the line. While this position provides the user improved access to the depthometer, the depthometer measurement output is presented to the user upside-down or in another position obstructing the ability of the user to read the output. Further, the depthometer must often be held by hand when positioned at the draw-works drum to obtain a reliable measurement, in violation of safety precautions. Safety precautions dictate that depthometers not be held by hand while in use to prevent the user from coming into contact with the line or other moving parts. Instead, a stand or tether and suspension eye is used to hold the depthometer in position.
To complicate this situation, most depthometers do not securely engage the main measurement wheel to the line. Consequently, movement of the line or a slight pull on the depthometer often disengages the wheel from the line resulting in unreliable measurements. When mounting a depthometer in proximity to the cable drum, placing the depthometer on a stand or securing it by tether is impractical. As the line is released from the drum, it travels left to right as well as transversely as the layers of wound line diminish. This causes the depthometer to constantly change position in relation to the movement of the line. When placed on a stand the depthometer continuously disengages from the cable due to the constant movement of the cable. Further, when the depthometer is tethered in proximity to the drum, the weight of the depthometer tends to pull the main wheel away from the line thereby disengaging the instrument from the line.
Other difficulties with conventional depthometers include open spokes on main measuring wheels which can injure fingers if placed too close in proximity; awkward installation procedures of the depthometer upon the line; damage to the counter due to accumulated debris in open gear components; and broken or damaged idler arms that hold idler wheels due to fatigue and misalignment.
What is needed is an improved depthometer that securely engages a line and provides accurate measurements, is easily and efficiently installed and uninstalled from the line, and accommodates improved safety measures.